It is clear from the statement of theorem REF  that the Hölder exponent \(\alpha \)  is not optimal. However, for the case \(q\ge 2\)  and \(f\in L^\infty _{\rm loc}(\Omega )\) , optimal value of \(\alpha \)  is mentioned in [1]} whereas the optimality of \(\alpha \)  in the subquadratic case is still unknown, even for the homogeneous operator case \(p=q\)  and \(s_1=s_2\) .
